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	California Quakes: Satellites See Reduced Risk By Craig Linder Special to SPACE.com posted: 08:00 am ET 19 August 2000

WASHINGTON, August 18 --- The Hayward fault has long been a source of concern for residents of the San Francisco Bay Area

(States News Service) WASHINGTON, August 18 --- The Hayward earthquake fault-line has long been a source of concern for residents of the San Francisco Bay Area.

More than 60 miles (97 kilometers) long, the fault traipses through some of the most heavily populated areas in Americas fifth-largest metropolitan area. Consider that the fault has not seen an earthquake in at least 300 years and the result is a combination that keeps seismologists and insurance actuaries awake at night.

Using satellite data, though, a team of scientists has found that there is a lesser chance of a major earthquake along the Hayward faults powerful northern branch than previously believed.

Radar interferometry image of the Hayward fault line

Researchers say that the news is good for residents along the faults northern branch, which runs near cities like Berkeley and Oakland, but that Californians shouldnt get too confident.

"It doesnt mean that people can go out and start unbolting their water heaters...but it does mean that the possibility of an earthquake on that part of the fault is lower," said Eric Fielding, a geophysicist at NASAs Jet Propulsion Laboratory (JPL).

Scientists at JPL worked with counterparts at the Lawrence Berkeley National Laboratory and the University of Californias Berkeley and Davis campuses.

The scientists used radar interferometry, data from global positioning satellites and an analysis of repeating earthquakes 6.2 miles (10 kilometers) below the surface to determine that the rocks within the faults northern branch were moving at about the same rate as the surface.

Interferometric synthetic-aperture radar, a relatively new technology, uses satellites to measure ground motion along faults. Mathematical analysis can determine how much the surface has moved, even over a series of years.

If the rocks within the northern branch had been locked into position while the surface moved, stress could have built up within the fault, which could have been released in the form of an earthquake.

In 1999, a team of seismologists estimated that there was a 28-percent chance of a major quake originating on the northern branch of the Hayward fault in the next 30 years. Preliminary data from this study allowed seismologists to revise that estimate to only 16 percent, Fielding said.

A major quake is one that registers a magnitude of 6.7 or higher on the Richter scale. The 1994 earthquake in Northridge, California, for instance, registered a 6.8 on the scale while the 1906 quake that devastated San Francisco scored an 8.3.

William Ellsworth, a geophysicist at the U.S. Geological Surveys Menlo Park, California office, said that the study was just one of several factors that contributed the revised earthquake forecast on the faults northern branch. Ellsworth was not affiliated with the Burgmann study.

Scientists collected data from two European Space Agency satellites, ERS 1 and ERS 2, to track the movement of the faults northern branch between 1992 and 1997. By using mathematical analysis software developed at JPL, researchers were able to pinpoint the amount of "surface creep" within a few millimeters.

Data from the satellites and software was checked against information from American global positioning satellites (GPS), which provide information that is reliable but not as precise as the data from the European satellites.

Roland Bürgmann, the studys lead author and an assistant professor of geology and geophysics at UC Berkeley, then combined that data with readings from what seismologists call "microquakes," tiny, almost imperceptible earthquakes that often occur just a few miles (kilometers) from Earths surface.

Though people cannot feel them, clusters of microquakes allow geologists to determine the amount of movement within a fault.

The faults northern branch moves at a rate of 0.2 to 2.8 inches (5 to 7 millimeters) each year, roughly the same as the movement of the surface.

Though JPL originally designed the analysis software used in the study for use creating topographical maps, Fielding said that, when combined with data from satellites, the package could have several uses.

In 1997, for instance, Fielding published an academic paper that used the satellite data and mathematical analysis to show that an oil field in Californias Central Valley was sinking at a rate of more than 1 foot (30 centimeters) a year.

"In the long term, the space-based technology that Bürgmann is using are extremely important," Ellsworth said. "This is a new way of seeing the Earth."

Fielding said that he and Bürgmann, who could not be reached for comment, plan similar studies on faults in Turkey, India and Tibet, adding that satellite data allows researchers to investigate faults in remote or unfriendly areas.

"We can apply it to places where there arent the ground measurements," he said.

Both JPLs Fielding and the Geological Surveys Ellsworth said that an American satellite dedicated to Earth science would be very useful for seismologists and geophysicists.

A satellite with a longer wavelength than the ERS satellites and a different orbit than the ERS polar loops is being considered as part of the National Science Foundations EarthScope initiative.

And that would make a useful tool even more valuable, Ellsworth said.

Space-based radar, he said, is "providing a new window into how the Earth works, and its encouraging that people like Bürgmann find clever new uses for this."

Bürgmanns study is published in Fridays edition of the journal Science.